Meter for alternating current



April 14, 1931. w. w. SCHERER METER FOR ALTERNATING CURRENT Filed Oct.

INVENTOR W/lL/AM m SC/B'EEEE QM @Zwq Hos ATTORNEY 30 Fig. 1 is a planPatented Apr. 14, 1931 wrnmnr w. scrim or osmium, csnrronma, me, orOAKLAND, csmrormm,

maniacal co arr-rm roe.

n conrona'rron orcamromrm n'rnaunrme curmmrr Application filed Octoberso, 1929. semi no. 408,453.

My invention relates to am neters for alternating current, andparticularly to what are known as line test sets by means of whichthealternating current flowing 1n a 1 line may be determined withoutopening the circuit.

An object of my invention is to provide a morerugged sensitive linetestset than has hitherto been feasible. .10 Another object of my inventionis to provide ameans of usin the simple DArsonval meter movement oralternating current measurement.

.Still anothergobject prxpyde a meter having a te perature coefiicient.I

A further object of 'my invention is to substantially zero rovide analternating current meter having a substantially uniform scale. I heinvention possesses numerous other some of forth mvendo not objects andfeatures' of advanta e, which, with the foregoing, will e se in thefollowing description of m tion. It is to be understood that $5 limitmyself to this disclosure of species of as I may adopt varlant emmyinvention,

bodiments thereof within the scopeoi the claims.

Referring to the drawings:

matic, showing the apparatus' used in one form of my invention. 1

Figs 2, 3 and 4 are schematic circuit diaams of various modifications ofmy invention.

Broadly considered, .the apparatus of my direct current aminvention comrises a meter, prefer-a 1y of the DArsonval type, which is connected tothe line in which current is to be measured, througha rectifier, I andwhich is provided with a shunt having a temperature coefiicient whichis',so proportioned to that of the rectifier as to give indications onthe meter substantially independent of temperature. In its preferreform, the apparatus, comprises a current transformer, the -secondar-winding of which supplies current meter, w relation to ich bears'apredetermined known the current flowing in the line.

of my invention is to able sector or wedge 7.

view, partly diagramsurface and a copper Itifier need not be describedin detail since contact rectifiers' of various kinds are well d creaseswith rising temperature,

to t e rectifier and Means are'provided for varying the ratio of linecurrent to meter current, in order to .assreuoa 'ro WESTERN ELECTRO-make the instrument cap ble of accurate readings over a widely varyingcurrent range, and these means preferably include a plurality of shuntsdiffering both in resistance and in their temperature coeflicient ofresistivity, so that the independence of temperature is maintained overthe entire range of the lnstrument.

'In its present preferred form my apparatus comprises a split corecurrent transformer such as those 1,141,642 to O.A. 1,489,665 to W. andJ. Schmid. This type of transformer consists of a toroidal core fihaving aremov- Th'e wedge is hinged.- to a spring pressed finger 8, bymeans of which it may be'withdrawn and the core slipped over the linewire'9, which carries the current to be measured, after which the wedgeis returned to the position shown, completing the magnetic circuit ofthe core.

The line wire 9 forms the primary of the transformer. Thejsecondarywinding 11 is wrapped around "approximately half of the core andcomprises many fine wire.

The winding .11 is connected by the leads 12 to the input described inPatent No. Knopp, or Patent No.

terminals of a rectifier 13. The latter is preferably of the contacttype,

J. Foster, W. W. Scherer,

turns of relatively rectification taking place between a copper oxidefilm. This recknown in the-art, all of them possessing substantiallysimilar-properties; All of these rectifiers possess a -neg'ativetemperature coeflicient of resistivity, cal resistance decreases withrising temperature. Their rectification ratio, i. e., the ratio of D. C.output to A. C. input, also dehowever,

that is, their electriand although the two eifects tend to neutralizeeach other, the net result is a decrease of output current as thetemperature rises. This effect may be termed a negative temperaturecoefficient of pei'formanee.

'Ilhis coefiicient is not a constant, but is greater at small loads thanatlarge ones.

It is preferable that the rectifier be of the so-called full-wave type.cuit rectifier shown in the figures has the advantage of requiring onlyasingle air of conductors connecting with the trans ormer winding, it isthe one that is usually desirable to use. The wellknown split winding te of rectifier circuit may, however, be siibstituted, and it is evenpossible to use the simple half wave rectifier in cases where lightnessis ofinore importance than accuracy in the device.

The bridge type rectifier has four rectifym contacts 14. These contactsare so'arranged that current can pass through the output circuit 15 ofthe rectifier in one direction only.

The rectifier is mounted one base '16, closely adjacent to a direct.current meter 17, preferably of the DArsonval type, which is connectedin series with the output terminals of the rectifier. Since-this meteris used-to measure current,'itimay properly be termed an ammeter. It ispreferable, however, that the meter be made with a windin having arelatively large number of turns'o? fine wire, that is, that it be morenearly' the typev of instrument that is usually used as :3, volt meterthan the ordinary direct current ammeter. An ordinary ammeter movement,or a millivoltimeter movement may be used, however, by properlymodifying the design of the transformer.

As is well known, a meter of this type will indicate the average valueof current passing through it and not the effective or root-mean-squarecurrent. As ordinary commercial current supplies approach very closely asine wave form, the reading of the meter, multi lied by the ratio of thetrans former, will indicate acurrent approximately 10% less thanthatwhich is actually flows ing in its windings. This is compensated forby su plying the meter with a scale whose readings are 1.11 times thetrue direct current valu Experiment has shown that when so calibrated,the readings of 'the meter with any wave form which is apt to be metwith in practice are not sensibly in error;

The meter is provided I with one or more shunts, each having a differentresistance, and each a different coefficient of resistivity. Theseshunts are wound non-inductively,

, comprising portions 18, 19,;and 21 (Figure 2g of a substantially zerotemperature coe cient material such as manganin, and

of a positive temperature coetficient material such as copper or nickel.

The materials are combined in 'difl'erent proportions in the differentshunts, the high resistance shunts having a higher proportion of hightemperature coeflicient material than As the bridge cirh ing the switcharms.

the low resistance shunts. A switch 22 is provided for connecting anyselected shunt across the input of the meter, which itself as a lowtemperature coefiicient. The switch is provided with a suitableindicator, which may be combined with a control knob 23 as shown in Fig.1.

In use the switch is set to connect that shunt across the meter whichwill ive the largest indication of current wit in the meter range. Asthe temperature rises the current delivered to the meter circuit falls,owing to the coeflicient' of performance of the rectifier, A shuntresistance diverts an increasing portion of this current through themeter itself, with the result that its indications are substantiallyconstant with temperature.

The term erature coefficient of the shunts is preferabl so chosen as tocorrect exactly for the per o'rmance coeflicient variation at about twothirds of the full scale readings of the meter on each scale. There willthen usually be a slight temperature error in opposite directions aboveand below this point, but this may readily be kept within a few percenton the low-range scale, and may be reduced to a small fraction of thison the higher ranges.

The various types of rectifier vary somewhat in their performancecharacteristics and it is therefore not practical to 've a formula forcomputing the shunts. he method which I prefer is to plot theperformance curve, draw a straight line which 7 will most nearlyrepresent this curve, and choose a value of temperature coeificientwhich will give the straight line correction. Other methods will suggestthemselves to those skilled in the art.

A modification of the invention is shown diagrammatically in Figure 3.Here a single shunt 26 26' is used across the meter, an the change incurrent range is accomplished by means of a switch 27 which connects tovarious taps 28 on the current transformer secondary 11'. The switch ispositioned on the meter base and controlled exactly as in the previousmodification. This arrangement has the advantage of loading therectifier uniformly for all ranges. Its disadvantages are a moreexpensive transformer, and the necessity of a multiconductorcablebetween the transformer and meter. I

Another modification, and for many purposes the most valuable uses acombination of the two methods. 11 this formfshown in Fi 4, a switch 31selects the suitable transformer tap while a second switch 32,mechanically interconnected therewith, simultaneously selects the propershunt. The interconnection 33- may be an insulated shaft but theconcurrent increase in common to both switches, or a link connect- Thisarrangement offers a means of using a single meter for a range ofcurrent measurements, say from 1000 amperes full scale to 12amperes fullscale, without using an excessive number of turns on the transformer andwith substantially the same percentage accuracy throughout.

All of the forms permit a more sensitive test'set to be constructed atlower cost than is possible with the usual transformer and A. C. meter.The meter scale, moreover, is substantially uniform, and sets can easilybe constructed with a full scale deflection of 12 amperes, whereas 35amperes has been the limit of stable operation with A. 0. meters.

It will be noted that the design of line test sets presents problems notencountered in ordinary current transformer practice, owing to thetransformer primary being limited to a single turn, and the magnetizingforces available being correspondingly low. The energy available for themeter operation is so small that the device becomes unstable in thelower ranges, and since the present invention operates on extremely lowenergy, it is especially applicable in this I field.

transformer having a split core and a secondary winding thereon, aplurality of tapped connections to the winding, a switch adapted toselectively engage the tapped connections, a rectifier in circuit withthe switch and the winding, a direct current.

indicating device in circuit with the rectifier, a plurality of shuntsad'acent the indicating device, and means or selectively connecting theshunts across the indicating device.

3. In combination with a rectifier whose ratio of input A. C. energy toD. C. output energy varies with the temperature thereof, a currenttransformer connected to the input of said rectifier, a meter connectedto the output of said rectifier, and means ineluding a shunt having atemperature coefficient proportioned to said A. C. to D. C.

ratio for changing the ratio of input transformer current toD. C. metercurrent and for maintaining the metenindications substantially changes.

4. In combination with a rectifier whose ratio of input A. C. energy toD. C. output energy varies with the temperature thereof, a currenttransformer connected to the input of said rectifier, a meter connectedto the independent of. temperature output of said rectifier, a pluralityof shunts for said meter having different resistances and temperaturecoefficients, and means for connecting a selected shunt across saidmeter for chang1n%theC ratio of input transformer current to metercurrent and for maintaining the meter indications substantiallyindependent of temperature changes.

5. In combination, a split core current transformer, a rectifierconnected to the output circuit of said transformer, a meter connectedto the out ut of said rectifier a plurality of shunts or saidmeter, eachof said shunts having a different resistance and a temperaturecoefiicient of resistivity proportioned to the temperature coefiicientof performance of the rectifier corresponding with the load imposed onthe rectifier b said shunt, and means for connectin a. se ected shunt incircuit with said rect' er.

. 6. In an alternating current measuring device, the combination of atransformer, a rectifier connected to the output of said transformer, ameter connected to the output of said rectifier, a plurality of shuntsfor said meter, and interconnected means for changing the transformerratio and the meter shunt.

7 In an alternating current measuring device, the combination of atransformer, a rectifier connected to the output of said transformer, ameter connected to the output of said rectifier, a plurality of shuntsfor said meter having different temperature coefficients of resistivity,and interconnected means for changing the transformer ratio and themeter shunt.

8. In combination with a tapped transformer and a rectifier the directcurrent out put of which varies with the temperature thereof and theload thereon, a meter connected to the output of said rectifier, aplurality of shunts for said meter havin ferent temperaturecoefficients, a switc goonnected to contact with any of a plurality oftaps on said transformer .and connected to said rectifier, a switch forconnecting any of said shunts across said meter, and a. mechanicalinterconnection between switches for varying them simultaneously. Intestimony whereof, I have hereunto set my hand. I

said

wiLLIAM w. SCHERER.

